The invention herein is concerned primarily with a method of and apparatus for toning electrophotographic film.
The field of toning has become important in recent years in connection with the increasing use of electrostatic techniques for reproducing drawings and documents and even more recently in the imaging of scenes directly, as the equivalent of conventional silver halide photography carried out under varying conditions of light and at high speeds.
According to the basic concept of electrostatic imaging, a photoconductive member is charged by corona or the like in darkness and its surface which has been charged is then illuminated by a light pattern of that which it is desired to reproduce. The charge is selectively dissipated by the projected light pattern, being permitted to remain at those surface increments of the photoconductive member which are not illuminated and being removed from those surface increments which are illuminated. As a general rule the degree of dissipation is directly related to the degree of illumination. In this manner there is produced a latent charge image of the pattern on the surface of the photoconductive member which slowly fades as the charge continues to leak off from the charged increments.
Without detailing the theory of operation of this technology, it should be noted in passing that the ability of a photoconductive member to retain a latent image with surface increments side by side having different degrees of charge is a function of its electrical anisotropy. The resolution capable of being achieved is a function of the size of the increments which in turn depends upon the material or materials from which the photoconductive member is made. The range of charges is a direct measure of the grey scale capabilities, and this in turn is controlled by the difference between light and dark resistivities. The speed with which a latent image can be formed is a measure of the gain of the photoconductive member or the response of the recombining of charges due to photons projected to the member.
The electrophotographic member which is disclosed in the above-mentioned application Ser. No. 434,699 is an example of a high speed, high resolution article which is well suited for use in connection with the invention herein.
According to the techniques of modern electrostatic technology, before the latent image has materially deteriorated, a comminuted powder of carbon, resin or the like is dusted, rubbed or flowed onto the surface of the photoconductive member. This material is called "toner" in whatever form it may be. The particles which make up this material are charged oppositely to the polarity of the charges which remain on the surface of the electrophotographic member by tribolectric or electrophoretic influence of other particles, solvents and the like. On this account, the toner particles will adhere in proportion to the degree of charge remaining on the charged increments after exposure and thus, the latent image is converted into a visible image.
Where the toner particles are suspended in a liquid, the toner may be viewed as a type of ink comprising toner particles and dispersant. Since wetting is usually desirable for liquid toner, low surface tension liquids such as hydrocarbons are typically used as dispersants. These have the added advantage of being insulative so that the charge of the particles is preserved. One such hydrocarbon which is an isoparaffinic hydrocarbon fraction is available in various grades from Exxon Company of Houston, Tex. as "Isopar. " The dispersant will electrophoretically affect the particles giving them a charge polarity, and in the case of Isopar it will be positive. This is ideal for the electrophotographic member of the above-mentioned application Ser. No. 434,699 when it is formed of the preferred photoconductive coating of ultrapure crystalline cadmium sulfide, either doped or undoped, since cadmium sulfide is an n-type material. The charge initially applied to the latter is achieved by electrons which are introduced into or below the surface so that the latent image is formed of negatively charged increments resulting from the fields apparent at the surface due to the presence of the electrons.
Two forms of electrostatic techniques are in wide spread use at the present time, one being known as xerography in which the photoconductive coating of the electrophotographic member is a layer of amorphous selenium carried on an aluminum drum. This first type of technique involves charging the selenium surface with a positive charge through corona and, after exposure, toning by means of dry toner particles mixed with resin. The method is known as "cascade toning" and is done in an enclosed chamber which confines the powder, recirculates it, etc. Various schemes involving the use of electrodes arranged opposite to the drum within the chamber are used to achieve increased efficiency during toning.
In xerography, the toned image carried on the drum is transferred to paper by pressure, and in some instances, the application of a low voltage bias, after which the drum is cleaned for reuse, while the transferred image is fused permanently in place by heat.
The second form of electrostatic techniques in use at this time is known as electrofax. The electrophotographic member in this case is a sheet of conductive paper having a coating of zinc oxide in a matrix of resin as the photoconductive layer. The imaging is effected by charging the surface negatively and, after exposure, the entire sheet is drawn through a liquid toner bath carrying toner particles in a dispersant. The toner adheres and is fused directly to the surface of the electrophotographic member whereby the latter member itself becomes the final copy of the document which has been reproduced.
The electrophotographic member of the co-pending application Ser. No. 434,699 affords a new electrostatic technique which has many advantages over known technology. As stated above, the photoconductive coating of this electrophotographic member is capable of being imaged at very high speed with great resolution. The member is formed of a substrate of polymeric organic transparent sheeting having a thin film layer of ohmic material about 300 to 500 Angstroms thick deposited on one surface and a thin film coating of the photoconductive material mentioned, preferably cadmium sulfide, sputter deposited on top of the ohmic layer. The thickness of the photoconductive layer is about 3500 Angstroms and it is a crystalline, dense, wholly inorganic, abrasion-resistant material which is quite transparent. Details of its characteristics are to be found in the said copending application Ser. No. 434,699.
Of the many functional characteristics of the material, three comprise its high speed, its high resolution and its ability to discharge to practically zero surface potential. Extremely efficient and highly sensitive toner with very fine particles is used in the imaging of this photoconductive material by means of which there is achieved the high resolution of which the material is capable. Absent residual voltage, backgrounds can be absolutely devoid of pigment, but in cases where even minute voltages remain, fog develops in the otherwise pigment-free background of resulting images. Conventional electrostatic reproduction has background fogging inherently because there is always a noise voltage, but there is practically no noise voltage in connection with the film of the copending application. High quality reproduction and imaging of scenes should thus be possible with such film but is not always achieved because of minute potentials attracting small amounts of toner particles.
The invention seeks to minimize fogging of this nature.
Another source of fogging is especially noted in cases where the toning is effected by a liquid toner flooded onto a film and then expected to evaporate or be swept from the surface. In this case, even if the toner is physically swept or decanted off the charged surface, there will usually be a liquid film of dispersant on the surface that contains toner particles that precipitate onto the surface when the dispersant finally evaporates. Such particles are not attracted to areas of very low or even zero surface potential, but literally "fall" onto the surface mechanically when they come out of suspension. Prior to this they are "floating" in the dispersant.
This is probably the most important problem to be solved by the invention. The solution is not linked to its application only to the electrophotographic film of copending application Ser. No. 434,699 but may be applied to any technique involving electrophotographic members where the charged surface is to be toned by liquid toner. The former case needs the invention more because the film of said copending application has greater capabilities which may not fully be realized but for this invention.
It is clear, of course, that the problem in the case of floating particles of toner can be solved by applying toner for the time needed to achieve a good image and then applying a clear rinse of the same dispersant which was used to make the toner suspension to wash the remanent toner away. Obviously, this is time consuming and expensive. In the case of a small hand-held camera to be used for scene imaging equivalent to candid photography, for example, the complexity of the mechanism to effect toning would be greatly increased if rinsing had to be included in the functions performed. This would increase size and expense.
By the invention it is feasible to "wash" the toner particles away from the film through the use of a highly simplified method using economical apparatus that will not materially affect the size or expense of a camera, or complicate its structure. This method involves no dispersant flow or movement.